Process for the manufacturing of the polymer compensation layer for LCD optical light shutter and the construction thereof

ABSTRACT

This invention solves the technical problem of compensating for the angular dependence of the contrast in optical devices comprising liquid crystal displays (LC optical light shutters, which operate on the principle of electrically controlled optical birefringence), with the aid of a compensation layer exhibiting optically negative birefringence, which enables the angular compensation of the LC layer birefringence in the state in which the LC molecules are homeotropically aligned (typical optically positive birefringence). The process for the manufacture of the optically negatively birefringent compensation layer is devised on the controlled spontaneous deformation of the polymer molecules during the polymerization procedure. The manufacturing process is feasible by the employment of known and well-controllable technical procedures, and enables the mass production of compensation layers. The invention solves the problem of manufacturing a compensation layer exhibiting the required optically negative birefringence, as well as the construction/manufacturing of the optical light shutter, which utilizes such a compensation film.

The object of the present invention is a process for the manufacture ofoptically negatively birefringent compensation layers, based on thecontrolled spontaneous deformation of the polymer macromolecules duringthe polymerization process, as well as the construction of an opticallight shutter utilizing such a compensation layer.

The technical field, dealing with this invention, is the compensatingfor the angular dependence of the intensity of the transmitted light inoptical apparatus equipped with liquid crystal light shutters. In thefollowing text, the liquid crystal shall be denominated LC (LiquidCrystal).

The technical problem, solved by this invention, is the enlargement ofthe viewing angle, and the compensation of the angular dependence of thetransmitted light intensity in the liquid crystal shutter elementsrespectively, either in autonomous elements in LCD (Liquid CrystalDisplay), or as components of optical apparatus comprising liquidcrystal filters. More precisely, there is involved the compensation ofLC optical light shutters, which function on the principle ofelectrically controlled birefringence, and are employed for modulatingthe intensity of the incident light in protective/safety devices, suchas LCD protective filters in welding helmets, in optical systems oflaser welding devices, and the like. The present invention enables theangular compensation of such a shutter in a typically closed state, inwhich the liquid crystal molecules are homeotropically aligned. Thisinvention solves the problem of manufacturing a negatively birefringentcompensation layer, which has the optical angle oriented perpendicularlywith respect to the surface of the optical light shutter, that isparallel to the homeotropically oriented molecules, as well as theconstruction and the manufacture of a LC electro-optical shutterelement, utilizing such a compensation layer.

The basic principles and natural laws, applied in the claimed processfor the angular compensation of LC light shutters, which is the objectof the present invention, are well-known and disclosed in severalpatents, such as Clerc et al. U.S. Pat. No. 4,001,028; Yamamoto et al.U.S. Pat. No. 4,984,874; Bos U.S. Pat. No. 5,187,603 and the like.Hitherto, there have been developed several successful technicalsolutions for the manufacture of compensation layers for LCD opticalshutters.

The first successful solution (#1) of the above mentioned technicalproblem was disclosed in 1989 by Uchida of the Tohoku University (Uchidaet al. SID 89 Digest, p 378–381), and in 1991 by Clerc of the Stanleycompany (SID Digest 91 p 758–761; U.S. Pat. No. 4,889,412; U.S. Pat. No.5,298,199). Both solutions were based on the two-dimensional mechanicaldeformation or stretching respectively, of certain thermoplastic polymermaterials during the thermal cycling through the glass phase transition.A few years later, a functionally similar result was achieved by Eblenet al. from the Rockwell Company (SID 94 Digest p 245–248; U.S. Pat. No.5,196,953) with the use of multi-layer thin-film oxide filtersexhibiting negatively birefrigent properties.

A new, technically highly interesting approach (#2) was developed byHarris and Cheng at the Akron University, USA, and somewhat later byShin-Tson-Wu from the Hughes company, USA (J. Appl. Phys. 76, 10, 1994;SID Digest 94, p 923–926; U.S. Pat. No. 5,344,916; U.S. Pat. No.5,580,950; U.S. Pat. No. 5,480,964). They disclosed that by means ofcentrifugal deposition of a thin layer of preimidized polyimides thelong molecular segments were preferably oriented in the plane of thedeposited layer, resulting in strong negatively birefringent propertiesof such layers, which represent an inexpensive and technicallyrelatively simple solution of the above described technical problem.

By all means, there also merit attention several technical solutionsdeveloped for computer terminals (#3) in companies, such as Nito Denko(Fujimura et al. SID Digest 91, p. 739, SID Digest 92, p 397–400; U.S.Pat. No. 5,245,456, . . . ), Sumitomo (Nakamura et al. U.S. Pat. No.5,061,042, . . . ) and (#4) Fuji Film (Mori et al., U.S. Pat. No.5,559,618; Mori et al., Display and Imaging 5, p 1 (96); Kamata et al.,U.S. Pat. No. 5,646,703; Mori et al. U.S. Pat. No. 5,583,679) and AkzoNobel (Picken et al. U.S. Pat. No. 5,382,648; U.S. Pat. No. 5,525,265, .. . ). The companies Nito and Sumitomo (#3) developed a multi-layercompensation film based on a one-dimensional deformation or stretchingrespectively; the Fuji company (#4) developed a polymeric discoticliquid crystal compensation sheet, the optical compensation propertiesof which could be altered with respect to the viewing/observation angleof the computer monitor; the Akzo Nobel company (#4) developed polymericholesteric liquid crystal compensation films on various polymers, whichwere ideally adapted for the angular compensation of STN computermonitors. All above-mentioned solutions are primarily intended for theangular compensation of the contrast in multi-plexially monitored LCmonitors, in which the homeotropic orientation of the LC molecules inthe selected pixel is not entirely feasible. For this reason, suchsolutions are neither technically nor economically appropriate for theabove-mentioned technical problem, that is the angular compensation ofLC optical light shutters, operating on the principle of electricallycontrolled birefringence, in a the state with highly homeotropicallyaligned LC molecules.

Interesting are also techniques for solving the above representedtechnical problem of angular compensation of LC optical light shutters,operating on the principle of electrically controlled birefringence,which are devised to utilize a plurality of liquid crystal cells (#5).Technical solutions based on the complementary orientation of twostandard TN cells (Twist Nematic—the rotation of the LC structure being90.degree.) adapted in such a manner, that their angular dependencescompensate each other, are used in manufacture by the majority ofmanufacturers (Optrel, Xelux, Jackson, . . . ) The technical solution ofthe ESAB company (Hornell, U.S. Pat. No. 4,240,709) is important aswell; it employs two TN cells, one of which is functioning in thepassive mode (a substantially reduced angular dependence of thecontrast), and the other in the active mode. In the >>passive<<TN cellthere is used a twist angle of the LC cell of 60.degree.–90.degree. TheSpeedglass company claims in the patent (Hornel et al., EP 706,674, WO9,529,428, U.S. Pat. No. 5,825,441) the use of two complementary TNcells, characterized by a rotation of the LC structures of less than90.degree.; in patent applications (Hornel et al., EP 805,661, WO9,715,254) and (Hornel et al., EP 858,305, WO 9,715,255) the samecompany amends its basic idea of exploiting two complementary TN cells,using a LC structure rotation of less than 90.degree., with the aid ofthe low-frequency control of LCD shutters, and an additional retardationlayer. With the utilization of several LC cells the angular compensationproblem is solved also in U.S. Pat. No. 5,515,186 (Fergasson et al.).The compensation of the angular dependence in such a mode is, however,relatively expensive, as each additional LC cell substantially increasesthe manufacturing costs of the light shutters.

It has to be recognized as well, that the problem of the angularcompensation of the layers of homeotropically oriented LC molecules isdirectly correlated with the problem of the angular dependence of thecrossed polarizers per se, which are the subcomponent of each LCelectro-optical shutter element, operating on the principle ofelectrically controlled optical birefrigence. In recent years, severalauthors have disclosed basic principles for the solution of thisproblem. The suggested solutions are based on the utilization of atleast two additional birefringent layers sandwiched between the LC cell,and the input and output polarizers. K. Ohmuro et al., Fujitsu company,Japan, (SID'97, 1, p 845) show that the problem is quite successfullysolvable by the utilization of three birefringent layers, two of whichare negatively birefringent, and have a fast optical axis orientedperpendicularly to the liquid crystal layer plane (parallel to thehomeotropically oriented LC molecules), and one of which is positivelybirefringent and has the optical axis oriented parallel to the liquidcrystal layer plane (perpendicularly to the homeotropically oriented LCmolecules!). The majority of other authors, such as H. Mori, P. Bos(IDRC'97, p M-88), J. Chen et al., Samsung company, Korea (SID'98Digest, p 315), devise their technical solutions on the utilization ofeffectively two additional negatively birefringent layers, one of whichhas the fast axis parallel to the polarization axis of the analyzer, andcauses a relative phase delay between the ordinary and the extraordinaryrays for ¼ of the light wavelength, and is denominated λ/4 plate. Thecombination of such a λ/4 plate, oriented in the above manner, with acorrespondingly thick additional birefringent plate exhibiting anoptical birefringence with the same sign, and an optical axis orientedin perpendicular direction with respect to the liquid crystal layer,that is parallel to the homeotropically oriented LC molecules,significantly improves the angular dependence of the light attenuationin crossed polarizers.

Although there is a slight similarity with respect to the deformation ofthe otherwise isotropic polymeric molecules, the claimed technicalsolution differs significantly from the hitherto optimal state of theart solutions:

-   -   Solution #1 Group: Uchida, Stanley, . . .    -   Solution #2 Group: Harris and Cheng, Hughes, . . .    -   Solution #3 Group: NITO Denko, Sumitomo Chemicals . . .        in the procedure applied to achieve the corresponding        deformation of the polymer molecules.

Uchida, Stanley (#1) and Nito as well as Sumitomo Chemicals (#3) employthe principle of two or one-dimensional stretching of a preformed, thatis a polymerized thermoplastic polymeric sheet. On the other hand,Harris and Cheng and Hughes (#2) utilize the method of alignment of longmolecular segments by means of a specific method of deposition of aprepolymerized polymer solution, such as the deposition of a preimidizedpolyimide on a rotating plate.

The present invention is based on the utilization of the volumecontraction of the polymer during the very polymerization process, sothat the monomer layer is during the polymerization in contact with thesurface of at least one rigid flat plate, or is preferably sandwichedbetween two layers of thermally and mechanically stable, rigidmaterials, for example glass. In such a manner, strains are generated inthe material in the plane, determined by the rigid boundary surfaces,and causing the corresponding deformation of otherwise isotropicmacromolecules. Since this deformation of the macromolecules is effectedduring the polymerization process, the cross-polymerization permanentlyfreezes the molecules in their deformed state, which is of the utmostimportance for the long-term and thermal stability of the birefringentfilms, manufactured in such a manner. The term “freezes” should beinterpreted in the sense of “hardens” or “becomes rigid” respectively.The very deformation of the macromolecules effected withcross-polymerization, endows the present invention with substantialadvantages in comparison with hitherto known methods of stretchingpreformed that is polymerized polymeric sheets. The latter has beenachieved by hitherto known processes either by means of a direct, moreor less one-dimensional mechanical stretching, of the type utilized inthe above mentioned technical solution #3 Group (Nitto, Sumitomo, . . .), or by means of a homogeneous, two-dimensional stretching of thethermoplastic polymeric sheet in the vicinity of the glass phasetransition of the employed polymer, which is characteristic for the #1Group of the above itemized known technical solutions—Uchida, Stanley, .. . . The claimed process does not require technically sophisticatedequipment for the controlled mechanical stretching. It enables massproduction, and furthermore, may be adapted in such a manner, that theobtained negatively birefringent polymer layer exerts simultaneously abonding action, that is the adhesion and the optical contact between theLCD cell, and the polarizing filter.

The present invention enables, in comparison with the technicalsolutions #2 a substantially reduced deformation of the molecules, thusrequiring thicker layers. The manufacturing process is, however, lessexpensive, more flexible and enables a significantly broader choice ofthe materials, as well as an improved precision and reproducibility. Animportant advantage is also the possible adaptation of the manufacturingprocess, enabling the simultaneous adjustment of the thickness of thecompensating polymeric layer to the individual LCD optical shutters,that is the adjustment as to the thickness and birefringence of theliquid crystal layer, the chosen polarizer, etc. This is not feasible inany of the three prior art processes, which require either anextraordinary exacting one- or multi-dimensional stretching (technicalsolutions #1 and #3), or an extraordinary sophisticated method for thedeposition of the compensating layer (technical solutions #2). Feasibleis only the mass production of the compensation sheet exhibiting adetermined selected retardation value, which is, however, notnecessarily optimal for the selected configuration of the LCD opticalshutter. The characteristics of the selected configuration areinfluenced primarily by the thickness and the refractive index of theliquid crystal layer, the selected polarizers, etc.

The present solution is completely different from the technicalsolutions #4 Group (Fuji, Akzo-Nobel, . . . ). The latter are indeed ofhigh technical quality, the methods are, however, highly sophisticatedand expensive. A mass production of a compensation layer with exactlydefined characteristics is involved, which in principle impedes theadjustment to the specific characteristics of the individual LCDshutters. At the same time it has to be emphasised that such materialscannot be simultaneously utilized for the bonding of the individualsubcomponents of the LCD shutter into a functional unity.

In comparison with hitherto known technical solutions, based on theutilization of several LC cells (#5), the claimed invention enables asignificantly less expensive and simpler application of the presentprocess. The employment of each supplementary LC cell renders in factthe product significantly more expensive; furthermore, other importantoptical characteristics, such as light scattering, are deteriorated.

The object of this invention is a process for manufacturing a polymericoptical compensation layer for LCD optical shutters, and thecorresponding construction of such a shutter. The process ought toenable the manufacture of an optically negatively birefringentcompensation layer, based on the controlled, spontaneous deformation ofthe polymeric macromolecules during the formation of the polymericlayer. The deformation of the polymeric macromolecules should resultfrom the volume shrinkage of the polymer volume during thepolymerization process, if the polymer layer is during thepolymerization continually in contact with at least one rigid platesurface. Preferably, however, it is sandwiched between two rigidsurfaces, under the provision, that the shrinkage of the polymer isunrestrained in the direction perpendicular to the surface plane. Theexpression “unrestrained” means, that it is in fact unrestrained, orthat the mechanical strains in the direction perpendicular to thesurface plane, are significantly reduced in comparison with the strainsin the layer plane. The optically birefringent polymer compensationlayer, manufactured in accordance with the present invention, ought toenable an easy adjustment to the specific characteristics of theindividual LCD optical shutters, by means of adjusting the thickness ofthe layer per se, and the polymerization conditions.

In addition to the compensation of the angular dependence of the opticalcharacteristics (light attenuation, . . . ) of the LCD optical shutter,the obtained optically negatively birefringent layer can also actsimultaneously as an optical contact adhesive, combining the individualsubcomponents of the LCD optical shutter into a functional unity.

According to this invention, the object is achieved in conformance withthe appended claims.

In the text bellow, the invention is exemplified and illustrated withthe aid of drawings, representing:

FIG. 1: a schematic depiction of a polymer macromolecule that is notdeformed, and a macromolecule deformed according to the claimed process;

FIG. 2: a schematic illustration of the process for manufacturing anoptically negatively birefringent polymer layer by the employment ofsoft spacers,

FIG. 3: a schematic illustration of the process for manufacturing anoptically negatively birefringent polymer layer by employing a two-stagepolymerization of the polymeric compensation layer, and hard spacers,which are removed prior to the second polymerization stage;

FIG. 4: a schematic representation of the construction of an angularlycompensated LC optical shutter, enabling the angular compensation of thebirefringence of the LC layer:

-   -   a—exemplifying mechanically stable polarizers;    -   b—exemplifying an additional glass plate, protecting the        mechanically sensitive polarizers,

FIG. 5: a schematic representation of the construction of an angularlycompensated LC optical shutter, enabling the angular compensation of thebirefringence of the LC layer, and the crossed polarizers.

The solution suggested by this invention resides in the spontaneousdeformation of the molecules of certain polymeric materials, such aspolyurethanes, polycarbonates, various polymeric materials for plasticlenses, such as allyl-diglycol-carbonate (ADC), laminating materials forpolarization films, such as cellulose-aceto-butyrate (CAB), andcellulose-triacetate (TAC) etc., materials respectively. The deformationis caused by the volume shrinkage of the material during the thermallyand UV triggered polymerization, under the provision, that during thepolymerization the monomer layer is in direct contact with at least onerigid plate surface, or is sandwiched between two layers of rigid,thermally and mechanically stable materials, such as glass.

Monomeric and prepolymeric materials, appropriate for this technicalsolution, are especially those that by virtue of their chemicalstructure, being already polymerized, during the linear stretching tendto become optically positively birefringent, having the main axis of therefractive index tensor, that is the optical axis, oriented in thedirection of the mechanical deformation, that is the stretching.

During polymerization the polymeric mass adheres to the boundary surfaceof the rigid plate. The spontaneous volume shrinkage of the polymerduring the polymerization results in the generation of mechanicalstrains in the material. This enables the free shrinkage of the materialin the direction perpendicular to the polymeric layer plane (in thethickness direction), that is the axis z, leaving only the strains onthe polymer layer plane, which are the axes x, y; in the direction ofthe axis z, however, the mechanical strains are substantiallydiminished. These strains cause a deformation of the otherwise isotropicmolecules of the polymer, to become flattened in the z axis direction,and they become uniformly stretched in all directions in the x, y planeof the polymeric layer, as represented in FIG. 1. The employment ofmaterials exhibiting the above-mentioned positive stretchingcharacteristics yields an optically negatively birefringent layer.

Triggering the polymerization at elevated temperatures can enhance thedeformation of the macromolecules during the polymerization process.Thus the differences between the thermal stretching of the polymer andthe rigid boundary surfaces increase the deformation of themacromolecules.

The selection of appropriate conditions for the technical procedure,especially the thickness of the layer, the chemical composition of theboundary plate surfaces, the polymerization time-velocity profile, etc.,endows the obtained polymeric layer with desired optically negativelybirefringent characteristics. These optical characteristics makepossible the angular compensation of the intensity of the transmittedlight for the homeotropically aligned LC molecular layer in the LCoptical shutter, which generally shows an optically positivebirefringence. The appropriate selection of the thickness of thecompensation polymeric layer enables the compensation of the angulardependence of the contrast/attenuation of the LC shutter. Thus, theoptical thickness of the compensation layer, which is the product of thebirefringence (Δn) and the thickness of the layer(d)→(Δn_(polymer)×d_(polymer)), is equal to the optical thickness of theliquid crystal layer, that is the product of the birefringence and thethickness of the layer (Δn_(LC)×d_(LC)).

Owing to the three-dimensional shrinkage during the polymerization, thecomplete process has to be performed in such a manner, that the strainsare generated only in the x, y plane of the polymer compensation layer,while the strains in the z direction (in the thickness direction),perpendicular to the surface, have to be minimal. The strains in thedirection of the z axis can be avoided by allowing the free variation ofthe thickness of the compensation layer, thus displacing at least oneboundary layer in the direction of the z axis during the polymerizationprocess.

This can be achieved in several ways, in conformance with three basicideas:

-   -   With the utilization of soft spacers, easily deformable under        pressure, between the boundary surfaces of the polymeric        compensation layer, by means of which the thickness of the        prepolymer layer is adjusted prior to the polymerization, and        simultaneously easily deformed by the action of forces generated        by the shrinkage of the material during the polymerization, thus        enabling a free shrinkage in the direction of the z axis.    -   By means of a gradual or multi-stage, preferably two-stage        polymerization, wherein the thickness of the layer between two        rigid boundary surfaces of the polymer mass (monomer and        prepolymer respectively), is at the beginning adjusted by hard        spacers, and the polymerization proceeds to the point only, at        which due to the viscosity and the surface activity the leakage        of the polymer mass is no longer possible. Then the rigid        spacers ensuring the corresponding thickness of the polymer        layer are removed, and the polymerization process is completed        in such a manner, that it enables the unrestrained shrinkage of        the polymer in the z direction perpendicular to the layer plane.    -   By pouring an appropriate prepolymer layer of controlled        thickness on a rigid support thus enabling the unrestrained        shrinkage of the thickness of the polymer layer in the z axis        direction. The process for the polymerization of the prepolymer        mass proceeds in contact with at least one boundary surface.

The following Examples are working embodiments, describing the processfor manufacturing a polymer layer exhibiting an optically negativebirefringence, as well as the construction of the LC optical shutter perse, with the utilization of such a polymer layer for the compensation ofthe angular dependence of the light attenuation.

EXAMPLE 1

A. Process for the Manufacture of an Optically Negatively BirefringentPolymer Layer

a) Single-Stage Polymerization—Soft Spacers

A polymeric, negatively birefringent compensation layer is formedbetween two rigid boundary surfaces as represented in FIG. 2. Thespacing between the boundary surfaces 1,2, preferably glass plateshaving a mutual distance equal to the thickness d, is filled up with themonomer or prepolymer mass 3 respectively. Appropriate are monomeric orprepolymeric materials, which by virtue of their chemical structure,already in their polymerized form tend to become optically positivelybirefringent during linear stretching, having the main axis of therefractive index tensor, that is the optical axis, oriented in thedirection of the mechanical stretching, for example a polyurethane-epoxycopolymer with a UV light activator. The spacing between the surfaces orthe prepolymer layer thickness respectively, is determined by means ofthe spacers 4, the so-called soft spacers, manufactured in a separateprocess from an appropriate material, for example a silicone gel havingappropriate dimensions and such a hardness as to remain undeformed atpressures resulting from the weight of the boundary plate having thesurface 1, as well as the surface tension in the non-polymerizedmaterial layer.

The layer is heated to an elevated temperature, for example 60° C. to80° C., and the resulting thermal expansion causes an additionalincrease of the strains, which are responsible for the deformation ofthe polymer macromolecules. An intensive, highly homogeneous UV lightemitted preferably by a 300–1000 W light source, predominantly in the UVspectral domain, is employed to trigger the polymerization of theprepolymer mass layer 3 in FIG. 2, which is terminated in an appropriateperiod of time. During the very polymerization procedure the temperatureof the polymeric layer may begin to decrease, so that the finalcross-linking, that is the cross-polymerization, can take place in amore or less cool material. In the course of the polymerization, thevolume shrinkage of the polymeric layer results in the generation ofstrains within the layer. The exertion of these forces causes thedeformation of the soft spacers 4, and enables the unrestrictedshrinkage of the polymer layer 3 in the direction perpendicular to thelayer plane or in the direction of the z axis respectively, while thestrains in the plane of the x,y axes remain, owing to the adhesion onthe surfaces of the rigid boundary plates 1,2. The polymermacromolecules thus acquire the typical deformation, as depicted inFIG. 1. The cross-polymerization permanently >>freezes<< the moleculesin their deformed state; this is of the utmost importance for along-term and thermal stability of the birefringent polymeric filmsobtained in such a manner.

The expression “freezes” is employed in the meaning of “hardens” or“becomes rigid” respectively. After the polymerization the rigidboundary plates are optionally removed. The obtained opticallynegatively birefringent polymer layer may be utilized as an autonomousoptically negatively birefringent element in various applications. Insuperior articles, for example in protecting welding filters, which arepredominantly multi-layer laminates made of different layers, such asinfrared light reflectors, polarization filters, and the like, theboundary layers may be provided by the individual elements of suchoptical assembly, and the above-described polymeric layer actssupplementary to the optical angular compensation of the homeotropicallyoriented LC molecules simultaneously as a bonding layer and as anoptical contact.

b) Two-Stage Polymerization—Hard Spacers

The process is initiated as in Example a), except that the spacingbetween the plates having the surfaces 1,2, is determined by the hardspacers 5 in FIG. 3; in conjunction with the aperture 9 in the uppersupport 8 it enables the illumination of the prepolymer layer 3 with UVlight. The polymerization is operated in two stages. The firstpolymerization stage is preferably activated at room temperature bymeans of a relatively weak UV light, advantageously provided by a 150 Wlight source 6 in the UV A spectral domain. The total visible area ofthe prepolymer mass layer 3 is illuminated as homogeneously as possible,and the illumination is interrupted immediately or within a few secondsafter the viscosity reaches the level, at which it impedes inconjunction with the surface tension the leakage of the partiallypolymerized mass.

The termination of the first stage of the partial polymerization isfollowed by the removal of the hard spacers 5 ensuring the properthickness of the polymer layer. Identically as in Example a), the layeris subsequently in the second stage, optionally thermally or by UVactivation, heated to an elevated temperature, preferably 60° C. to 80°C. to enhance additionally by thermal stretching the strains responsiblefor the deformation of the polymer macromolecules. The intense, highlyhomogeneous UV light, which is preferably provided by a 300–1000 W lightsource predominantly in the UV A spectral domain, triggers the processof the final polymerization of the prepolymer mass that is accomplishedin some ten seconds. During the very polymerization process thetemperature of the polymer layer may begin to decrease, so that thefinal cross-linking occurs in the more or less cool material. Thecross-polymerization permanently freezes the molecules in the deformedstate, which is of the utmost importance for a long-term and thermalstability of the birefringent polymer films manufactured in such amanner. Identically as in Example a), the rigid boundary plates areoptionally removed, and the obtained optically negatively birefringentpolymer layer may be utilized as an autonomous, optically negativelybirefringent element in various applications. In superior articles, forexample in protective welding filters, which are predominantlymulti-layer laminates made of different layers, such as infrared lightreflectors, polarization films and the like, the boundary layers may beprovided by the individual elements of such an optical assembly. Theabove-described polymer layer functions as an optical angularcompensation layer for the homeotropically oriented LC molecules, aswell as simultaneously as a bonding layer and an optical contact.

Optionally the polymer mass is in the first stage illuminated at themargins only of the polymer layers, outside the usable, namely is theviewing area of the later optically negatively birefringent compensationlayer. The polymerization or the illumination with UV lightrespectively, is interrupted as soon as the hardness of the material inthe illuminated region increases to the point, at which the polymerizedareas function as soft spacers.

The processes for the manufacture of the polymer compensation layer forLC optical shutters having the optical axis perpendicular to its surfaceplane, are characterized in:

-   -   that the monomer or prepolymer mass is poured on a rigid        boundary surface, and the polymerization of the prepolymer mass        proceeds in contact with at least one rigid boundary surface;    -   that the monomer or prepolymer mass is poured between two rigid        boundary surfaces separated by soft spacers, which are deformed        under pressure, in such a way that the polymerization of the        prepolymer proceeds in contact with the two rigid boundary        surfaces;    -   that the monomer or prepolymer mass is poured between the two        rigid surfaces divided by hard spacers, and the polymerization        of the prepolymer mass in contact with the two rigid boundary        surfaces proceeds at first only to the level, at which the        viscosity of the mass is increased to the point at which it does        not leak out, whereupon the hard spacers are removed and the        polymerization process proceeds to the end.

The expression “poured” is to be understood in the broadestinterpretation, which means that there may in fact be poured on a rigidboundary surface, or sandwiched, with the aid of the surface tensioneffect, between two rigid boundary surfaces, or poured on one rigidboundary surface, and the other rigid boundary surface is applied to thelayer after the pouring.

The polymerization process is either thermally or UV activated, and isoperated at elevated temperature, which is at least somewhat lower thanthe glass phase transition of the polymer. In most cases the attainedoptical birefringence of the polymer layer may be optionally decreasedin a suitable, controlled way, by means of reheating the polymer layerin the vicinity of the glass phase transition of the polymer. Thepolymerization is at least at the beginning activated by means of UVlight. The activation with the UV light 6 proceeds optionally in twostages, so that in the first stage, when the thickness of the layer isdetermined by the hard spacers 5, it proceeds to the level only, atwhich the increased viscosity stabilizes the thickness of the layer 3 tothe point at which the hard spacers are removable, and so the next phaseof the UV-activation enables the operation of the polymerization withoutthe generation of strains in the direction perpendicular to thecompensation layer 3.

B. The Construction of an Angularly Compensated LCD Optical Shutter

The process for manufacturing an optically negatively birefringent layeraccording to this invention enables an entirely novel constructionalsolution of manufacturing angularly compensated optical LC lightshutters. Such a layer performs the basic function of compensating forthe angular dependence of the optically positive birefringence of thehomeotropically oriented layer of the liquid crystal, as well as thebonding of the subcomponents into a mechanically and optically uniformassembly, and further ensures the optical contact, which is not feasibleby any of the prior art technical solutions.

The construction according to this invention is performed in such amanner, that one or both polarizers, which in conjunction with the LCcell form the optical shutter, are not directly laminated to the LCcell, but to one of the outer protecting layers, for example to a glassor IR/UV filter, and subsequently, by means of the optically negativelybirefringent polymer layer combined according to the invention with theLC cell into a mechanically and optically uniform assembly.

In general, two concepts for the optical compensation of the LC lightshutters are involved.

a. The compensation of the angular dependence of the optically positivebirefringence in a homeotropically oriented LC layer in a LC cell.

b. The compensation of the angular dependence of the optionally positivebirefringence in a homeotropically oriented LC layer in a LC cell, aswell as the angular dependence of polarizers crossed perpendicularly toeach other and forming in conjunction with the LC cell an LC opticalshutter.

Ad a. In most cases, the compensating for the angular dependence of theoptically positively birefringence of the homeotropically orientedliquid crystal layer in the LC cell 13, fulfils the technicalrequirements in a completely sufficient manner, and thus theconstruction of such a LC optical shutter represented in FIG. 4, issimplified. Inasmuch as the outer layers of the polarization film maywithstand the mechanical pressure in the x,y plane of the polarizationfilter, generated during the above-described polymerization process(according to the working Examples Aa and Ab) of the opticallynegatively birefringent layer 3, the preferred and most appropriateconstruction is at first the bonding, by means of a standard isotropicadhesive 17, of the polarizer 15 with one of the outer protective layerscorresponding to the boundary surfaces 2, such as glass, and a LC cell13, the polarizer/analyzer 12 and the IR/UV reflector 11, to yield twoautonomous assemblies. The obtained components of the LC shutter arecombined in accordance with the above-described process into a unity bymeans of bonding with an appropriate polymer layer 3, such as apolyurethane-epoxy copolymer with an UV light activator. Such aconstruction of the optical shutter is represented in FIG. 4 a.

The conditions in the manufacture of the shutter should be strictlycontrolled, in conformance with the described process of manufacturingthe optically negatively birefringent polymer layer, in respect of theworking Examples Aa and Ab, as well as in respect of the mechanicalconstruction, which has to secure a strictly defined and controlledthickness of the layer, for example 300 μm, as well as thepolymerization conditions: temperature profile, UV light illuminationprocedure, so that the optically negative birefringence is induced inthe polymer layer 3 during the polymerization. The appropriateperformance of the whole process, that is with soft spacers or themulti-stage polymerization enables the perpendicular orientation of theprincipal axis of the refractive index tensor with respect to the layer.If simultaneously its optical thickness is such, that the integraloptical thickness, that is the multiplication product of thebirefringence and the thickness of the layer (Δn_(polymer)×d_(polymer)),of this layer and the optical thickness of the two polarization filters,which themselves exhibit slightly optically negatively birefringentcharacteristics, is equal to the optical thickness of thehomeotropically oriented liquid crystal layer, that is themultiplication product of the birefringence and the thickness of thelayer (Δn_(LC)×d_(LC)), the requirement for the compensation of theangular dependence of the contrast/attenuation of the LC shutter isfulfilled. In addition to this basic function the layer also combinesthe two subcomponents into a mechanically and optically uniformassembly, and ensures the optical contact.

If the outer layers of the polarization filters do not withstand themechanical pressure in the x-y plane of the polarization filter,generated during the above-described polymerization of the opticallynegatively birefringent layer, the preferred and most suitableconstruction is represented in FIG. 4 b. In this case the opticallynegatively birefringent compensation polymer layer 3 is formed betweenone of the surfaces of the LC cell 13, and an additional, preferablyglass plate 18, which is an effective protection for thepolarizer/analyzer 15. This manufactured assembly consisting of the LCcell 13 and the optically negatively birefringent polymer layer 3,sandwiched between one of the outer glasses of the LC cell 13 and theadditional, preferably glass plate 18, is laminated on both outersurfaces with the crossed polarizers 12 and 15 by means of a standardisotropic adhesive 17, such as a silicone gel and the like. This opticalshutter exhibits significantly improved characteristics in comparisonwith a standard LC optical shutter.

It is understood, that it is optionally possible to combine the twopolarizers with the LC cell by means of the optically negativelybirefringent polymer compensation layer in conformance with theinvention, instead of the standard optically isotropic adhesive. In thiscase, however, the sum of the optical thicknesses of the two polymerlayers and the optical thickness of the two polarization filters, whichthemselves exhibit slightly optically negatively birefringentcharacteristics, has to be equal to the optical thickness of thehomeotropically oriented liquid crystal layer, that is themultiplication product of the birefringence and the thickness of thelayer (Δn_(LC)×d_(LC)); the requirement for the compensation of theangular dependence of the contrast/attenuation of the LC shutter is thusfulfilled.

Ad b. The constructions of the angularly compensated LC optical shutter,described in the working Example a), solve exclusively the problem ofthe angle-dependent birefringence of the layers of homeotropicallyoriented LC molecules. They do not, however, solve the problem of theangular dependence of the crossed polarizers themselves, which are thecomponents of any LC electro-optical shutter element operating on theprinciple of electrically controlled optical birefringence. Theutilization of an optically negatively birefringent layer, that is theobject of the claimed invention, enables the optimal constructionalembodiment, and at the same time ensures the angular compensation of thebirefringence of the homeotropically oriented LC molecules, as well asthe compensation of the angular compensation of the crossed polarizerswith the use of an additional λ/4 plate. Since the optical retardationlayers are significantly dependent on the wavelenght of the light, is anideal example for the utilization of the above-mentioned angularcompensation principle the protective welding filter requiring for thesupplementary eye protection from UV and IR light, an additional filterfor the yellow-green light of the wavelenght approximately 550 nm. FIG.5 depicts the construction of such a shutter, which is a multi-layerlaminate. It is composed of: a thin-layer IR/UV filter 11 that at thesame time ensures the protection from the harming IR and UV light, aswell as the selective transmittance of the light having a wavelenght of550 nm, corresponding to the λ/4 plate, of two crossed polarizers 12,15, and a LC cell 13 in the closed state with homeotropically orientedmolecules, exhibiting an optical birefringence Δn_(LC) and having athickness d_(LC), a polymer layer 3 of a thickness (L) exhibiting anoptically negative birefringence Δn_(L) and having an optical axisdirected perpendicularly to the layer itself, a λ/4 plate 19 for thelight of a wavelenght of 550 nm, exhibiting an optically positiveoptical birefringence, the slow axis of which is parallel to thepolarization transmission axis of the polarizer/analyzer 15 and theprotective glass surface 2. The basic idea is, that the opticalthickness (Δn₁×l) of the optically negatively birefringent polymer layer3 in this case does not adapt directly to the optical thickness of theliquid crystal layer, as in the working Example Ba. In this case it hasto be secured, that the thickness (L) of the optically negativelybirefringent layer 3 of the optical adhesive layer in conjunction withthe negative birefringence of the two polarizers is such, that thedifference of the refractive indices for the ordinary and theextraordinary rays (Δn_(L)) is such, that the difference of the opticalpaths for the ordinary and the extraordinary rays (Δn_(L)×L) is smallerthan the difference of the optical paths for the ordinary and theextraordinary rays in the LC cell 13 with homeotropically orientedmolecules (Δn_(LC)×d_(LC)). In such a way the optically uncompensatedpart of the LC layer 13 operates as an optically positively birefringentplate, the optical axis of which is perpendicular to the axis of the λ/4plate 19, while the difference of the optical paths in this part of thelayer is such that together with the λ/4 plate 19 it ensures the angularcompensation of the two polarizers 12,15 of the LC light shutter.

The preferred constructions are as follows:

-   -   At least one of the polarizers 12,15 is laminated with an        isotropic contact adhesive to the outside protective plate 2 of        the light shutter instead of directly to the LC cell, so that        there is a polymer layer 3 between at least one of the        polarizers 12,15, and between at least one of the boundary        surfaces of the LC shutter. According to this invention, the        polymerization is performed in such a manner that the polymer        performs the function of combining the subcomponents of the LC        shutter into a functional unity, while at the same time ensuring        the angular compensation of the LC shutter in the state, in        which the molecules are homeotropically oriented with respect to        the boundary surfaces of the LC shutter.    -   The optically negatively birefringent polymer layer 3 is        deposited on one of the boundary surfaces of the LC cell 3, so        that said polymer layer is sandwiched between the surface of the        LC cell 13 and the rigid transparent, preferably glass plate 18.        Two crossed polarizers 12, 15 are laminated with an isotropic        optical adhesive 17 on each of the boundary surfaces of such        assembly, and the protective outer glass plate 2 and an IR/UV        filter 11 are laminated to these polarizers by means of an        isotropic optical adhesive 17 as well.    -   The construction of a LC light shutter with the use of an        optically negatively birefringent polymer adhesive layer 3,        optionally comprising an additional optically negatively        birefringent layer between the polarizer and the LC cell. The        thickness of the layer 3 corresponds to the requirement for the        λ/4 plate 19, the slow axis of which is parallel to the        polarization (transmittance) axis of the polarizer/analyzer 15,        wherein the thickness (L) of the optically negatively        birefringent polymer adhesive layer 3 is such, that the        difference between the refractive indices for the ordinary and        the extraordinary rays (Δn) is such, that the difference of the        optical paths for the ordinary and the extraordinary rays        (Δn_(L)×L) is smaller than the difference of the optical paths        for the ordinary and the extraordinary rays in the LC cell with        homeotropically oriented molecules (Δn_(LC)×d_(LC)). Thus the        optically negatively birefringent polymer adhesive layer 3, the        optically negative birefringence of the polarizer 12, and the        optically uncompensated part of the LC layer in the LC cell 13        operate as a optically positively birefringent plate, the        optical axis of which is perpendicular to the axis of the λ/4        plate 19, while the difference of the optical paths in this part        of the layer is such that in conjunction with the λ/4 plate 19        it ensures the angular compensation of the two crossed        polarizers in the LC light shutter.    -   The construction of a LC light shutter with the use of an        optically negatively birefringent polymer adhesive layer 3, and        as alternative optionally an additional optically negatively        birefringent layer sandwiched between the polarizer and the LC        cell, the thickness of which corresponds to the requirement for        the λ/4 plate 19, the fast axis of which is parallel to the        polarization axis of the polarizer/analyzer 15. The        thickness (L) of the optically negatively birefringent polymer        adhesive layer 3 is such, that the difference of the optical        paths for the ordinary and the extraordinary rays (Δn_(L)×L) is        greater than the difference of the optical paths for the        ordinary and the extraordinary rays in the LC cell 13 with        homeotropically oriented molecules (Δn_(LC)×d_(LC)), in such a        manner that with the LC layer and the optically negative        birefringence of the polarizer 12, the optically uncompensated        part of the optically negatively birefringent polymer adhesive        layer 3 operates as an optically negatively birefringent plate,        the optical axis of which is perpendicular to the axis of the        λ/4 plate 19, while the difference of the optical paths in this        part of the layer is such that in conjunction with the λ/4 plate        19 it ensures the angular compensation of the crossed polarizers        12,15 in the LC light shutter.

It should be emphasized, that the described Examples represent only afew feasible working embodiments of the claimed invention. Variousmodifications and variations can be made within the scope of thisinvention, such as the utilization of a λ/4 plate exhibiting anoptically negative birefringence, the employment of an additional glassplate, which mechanically separates/protects the polarizer and the λ/4plate respectively, from the optically negatively birefringent polymercompensation layer, and the like.

1. A process for manufacturing an optical negative birefringent layerconsisting of a monomer material or a prepolymer material and having anoptical axis perpendicular to a surface thereof, said process comprising(1) pouring the monomer material or the prepolymer material (a) over arigid substrate surface, (b) between two rigid substrate surfacesseparated by deformable spacers, or (c) between two rigid substratesurfaces separated by non-deformable spacers, to form a material layer;(2) polymerizing said material layer at an elevated temperature which islower than a glass phase transition temperature of the polymerizedmonomer material or the prepolymer material, such that for (a) or (b)said material layer is fully cured, and for (c) said material layerpolymerizes at room temperature to a first level at which viscosity ofthe monomer material or the prepolymer material is increased to a pointthat the monomer material or the prepolymer material does not leak outfrom between the substrate surfaces, followed by removal of thenon-deformable spacers, and completion of the polymerizing at saidelevated temperature, so that for (a), (b) and (c) the material layerunrestrainably shrinks in a direction perpendicular to the substratesurface or the substrate surfaces, wherein said polymerizing of saidmaterial layer is conducted in such a manner to provide a spontaneousdeformation of molecules forming the monomer material or the prepolymermaterial, which is induced by an anisotropic mechanical strain due toshrinking the material layer in contact with and parallel to thesubstrate surface or the substrate surfaces, which is permanentlyfrozen-in by cross-linking polymerization and results in strain-inducednegative birefringent properties in the material layer; and (3) coolingsaid material layer following said polymerizing to room temperature. 2.Process according to claim 1, wherein said polymerizing is thermallyactivated at the elevated temperature which is lower than said glassphase transition temperature, and optical birefringence is reduced byreheating the material layer polymerized to a temperature approximatelyequal to a glass phase transition temperature of the polymerizedmaterial.
 3. Process according to claim 1, wherein said polymerizing isactivated at least initially by UV light.
 4. Process according to claim1, wherein activation of said polymerizing is by UV light and saidpouring of said monomer material or said prepolymer material is inaccordance with (c), said activation by said UV light comprising a firststage and a second stage wherein the first stage is to a level allowingremoval of the non-deformable spacers, and the second stage is tocompletion substantially in absence of mechanical strains in a directionperpendicular to the material layer polymerized.
 5. A process formanufacturing an optical negative birefringent layer which is an OpticalCompensation layer (OCL) for angular compensation of phase retardationof a transmitted light through a liquid crystal layer (LCL) and twopolarization filters, said OCL consisting of a monomer material or aprepolymer material and having an optical axis perpendicular to asurface thereof, with said OCL and said LCL having an optical thicknesswhich is a product of birefringence (Δn_(OCL), Δn_(LCL)) and thicknessof the layer (d_(LCL), d_(OCL)) respectively, and said two polarizationfilters having an optical thickness (p₁, p₂), respectively, said processcomprising (1) pouring a predefined mass of the monomer material or theprepolymer material (a) over a rigid substrate surface, (b) between tworigid substrate surfaces separated by deformable spacers, or (c) betweentwo rigid substrate surfaces separated by non-deformable spacers, toform a material layer; (2) polymerizing said material layer at anelevated temperature which is lower than a glass phase transitiontemperature of the monomer material or polymerized the prepolymermaterial, such that for (a) or (b) said material layer is fully cured,and for (c) said material layer polymerizes at room temperature to afirst level at which viscosity of the monomer material or the prepolymermaterial is increased to a point that the monomer material or theprepolymer material does not leak out from between the substratesurfaces, followed by removal of the non-deformable spacers, andcompletion of the polymerization at said elevated temperature, so thatfor (a), (b) and (c) the material layer unrestrainably shrinks in adirection perpendicular to the substrate surface or the substratesurfaces; wherein said polymerizing of said material layer is conductedin such a manner to provide spontaneous deformation of molecules formingthe monomer material or the prepolymer material, which is induced by ananisotropic mechanical strain due to shrinking the material layer incontact with and parallel to the substrate surface or the substratesurfaces, which is permanently frozen-in by cross-linking polymerizationand results in strain-induced negative birefringent properties in thematerial layer; and (3) cooling said material layer following saidpolymerizing to room temperature; wherein mass of said monomer materialor the prepolymer material and thus thickness of the OCL (d_(OCL)) isselected, such that a sum of optical thicknesses of a fully cured OCLand the two polarization filters equals optical thickness of the LCL(Δn_(OCL)×d_(OCL))+p₁+p₂=(Δn_(LCL)×d_(LCL)).